It is reasonably well known that welding significantly reduces the corrosion resistance of stainless steels. For example, in the text entitled "Corrosion and Corrosion Control--an Introduction to Corrosion Science and Engineering" Third Ed., H. H. Uhlig, et al., it is taught that when austenitic stainless steels have been welded and then subjected to corrosive environments, failure of the austenitic stainless steel weld (called weld decay) occurs in a zone slightly away from the weld rather than at the weld itself. It is taught that, under certain conditions, rapid quenching from high temperatures can aid in improving corrosion resistance.
The use of silicon as a protective coating is fairly well known. For example, in an article appearing in the Journal of Material Science, Vol. 26, at pp. 9445-52 (1991) entitled "Chemical Vapor Deposition of Silicon Onto Iron: Influence of Silicon Vapor Phase Source on the Composition and Nature of the Coating", it was taught that Armco iron could be treated with a gas mixture of argon, silane and hydrogen. It was taught that this gas mixture, when applied at a range of 750.degree. to 1100.degree. C., provided a non-porous and adherent solid solution with a maximum silicon content of 6% (by weight). It was hypothesized that such a mixture lead to the nucleation of Fe.sub.3 Si, the growth of which occurred from and around the open porosity of the coating.
It is known that stainless steel, and particularly 316 L stainless steel, is exceedingly corrosion resistant. In fact, tubing made of 316 L stainless is the conduit of choice in microelectronics and semiconductor manufacturing facilities where such tubing is required for the delivery of corrosive gases. The stainless steel tubing is made even more corrosion resistant by conducting various well known surface treating operations such as electropolishing, oxygen passivating and bright annealing. Electropolished tubing is currently the preferred conduit in terms of impurity absorption and corrosion resistance.
It is also known from EP-A-512782 a method of surface passivating stainless steel by flushing the surface of stainless tube with argon and baking said tube at a temperature between 250.degree. C. to 500.degree. C. during at least four hours.
Stainless steel tubing is generally assembled by welding employing an orbital tungsten inert gas (TiG) or Metal Inert Gas (MiG) system. In such a process, an arc is established between a non-consumable electrode such as tungsten or a tungsten alloy and the L stainless steel based metal. Like all other welding processes, the necessary heat is supplied by an arc which is initiated and maintained by the electrical energies supplied by a power source. A constant current power source, either AC or DC or combined AC-DC is used with the process.
A torch of an appropriate design connected to hoses and cable holds the electrode, dispenses the inert gas, cools the torch head, if coolant is used, and supplies energy to the electrode. The arc is highly concentrated and can produce temperatures up to 19,425.degree. C.
The weld puddle and arc area are protected against contamination from oxygen and nitrogen contained in the atmosphere by a shield of inert gas. The inert gas is generally a member selected from the group consisting of Ar, He, H.sub.2 a mixture of Ar and He, a mixture of He and H.sub.2, and mixtures thereof. Other contaminants like moisture, dust, metallic particles in suspension, etc., are also prevented from contacting the metal in its liquid state or during the solidification process.
Despite the fact that stainless steel, and particularly 316 L stainless steel, is known to be extremely corrosion resistant, corrosion sensitivity increases dramatically in both the liquid and gas phases within a zone including and surrounding the welding bead. As a result, conduit failures have been commonplace despite the use of 316 L stainless having been surface treated. The weld joints fail long before the stainless steel tubing suffers any ill effects from its contact with liquid and gaseous corrosive fluids.
It is thus an object of the present invention to provide a process for rendering such weld joints resistant to attack by corrosive gases and liquids.
It is yet another object of the present invention to provide a process for rendering the weld joints of 316 L stainless steel resistant to attack by corrosive gases and liquids in a convenient manner without significantly adding to the cost or complexity now experienced in carrying out ordinary orbital tungsten inert gas welding.
These and further objects would be more readily apparent in considering the following specification and appended claims.